Replace boost hash usage with TfHash in pxr/base/vt

[nvmkuruc]

Description of Change(s)

Requires #2173, #2304, #2298

To remove the dependency of pxr/base/vt on boost's hashing functions

• VtDictionary defers its hashing to TfHash's default std::map hashing
• VtArray implements a TfHashAppend overload to take advantage of the hashability of its contiguous elements. hash_value has been modified to use this via TfHash.
• Remove explicit hash_value support in hash.h as TfHash will fallback to hash_value if no explicit TfHashAppend is defined. This may change the order of which hash implementation is chosen (now preferring TfHashAppend to hash_value) but the implementations should not vary for most types.
• testVtCpp now provides test coverage for VtArray hashing
• Since TfHash supports hashing via std::type_index and std::pair, an explicit implementation hashing the type index pair is no longer necessary

Fixes Issue(s)

-#2172 (additional PRs forthcoming)

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[sunyab]

Filed as internal issue #USD-7931